1. Field of the Invention
This invention relates to the extraction of carboxylic acids from solutions containing them. More particularly, this invention relates to polymer conjugates of polyethylene glycols or oxides with polyethyleneimine or polypropylenimine for use in such extraction processes, and the extraction processes.
2. Description of Related Art
Carboxylic acids are promising intermediates in a bioprocessing complex, because the oxygen of the biomass is placed in a form that is useful for further reaction with many other products. Lactic, propanoic (propionic), succinic, and citric acids among others fall in this group, and are, or have been, manufactured via fermentation.
Lactic acid has been produced via fermentation since 1881, and in 1990 about 20,000 tons/year were obtained fermentatively, accounting for half of the lactic acid world production, the remainder of the lactic acid being produced in chemical processes. The main destinations of lactic acid are food and pharmaceutical industries, but an emerging polymer industry is increasing the demand for the production of lactic acid to be used as monomers in the synthesis of thermostable and biodegradable plastics.
The main drawback in the fermentative production of carboxylic acids is that, during their production, an accumulation of the acid in the medium results in inhibition of cell growth as well as production of the acid itself. Continuous high productivity bioreactors with cell recirculation systems facilitating cell separation and resulting in high cell densities may be used to improve the performance of fermentation-based processes and thus make them economically more attractive. Drawbacks, such as limited diffusion of the nutrients and release of cells, have been reported in systems with cells entrapped within porous matrices or by microencapsulation. Centrifugation and filtration are useful systems for cell separation, but result in high stress levels that decrease the performance of the fermentation process when used as cell recirculation systems.
These disadvantages are avoided when cells are entrapped in a two-phase emulsion obtained by mixing a water solution with a suitable organic solvent, such that the cells grow in the aqueous phase and the product can be removed from the organic phase. However, organic solvents are often injurious and even toxic to the cells.
The use of a two-phase system based on aqueous solutions of two polymers, as an extractive fermentation system, provides a more biocompatible environment for the cells than aqueous-organic two-phase systems. The uneven partitioning of cells in aqueous two-phase systems (ATPSs) allows the implementation of a stress free cell recycling process.
From economical considerations the use of cheap raw material for the production of lactic acid must be considered. Starchy material, such as potato starch, tapioca starch, wheat starch, or, more suitably, corn starch, can be hydrolyzed and used as a substrate for the fermentation. A further advantage is that starch might be used as phase-forming polymer. Thus, having a double function as feed stock and phase forming polymer, the use of starch makes the process certainly much more interesting from an economic point of view.
Recovery of lactic acid is impeded by the complex nature of fermentation broths, by dilute product streams, and by the physico-chemical properties of lactic acid itself, which can not be distilled and which is difficult to crystallize. At present, the most widely used process for the recovery of lactic acid involves precipitation and filtration of the calcium salt of the acid. Treatment of the precipitate with sulphuric acid leads to preferential precipitation of CaSO.sub.4, which is filtered off. Concentration by water, evaporation, and purification by crystallization are used to achieve the final product specifications. This method has the disadvantage of irreversibly consuming the base used for the titration of lactic acid during the fermentation and the sulphuric acid, leaving as waste large quantities of sulphate, which gives rise to disposal problems.
Alternative techniques, such as extraction and sorption, have been developed. Extraction of lactic acid can be performed by three different extractant categories: (i) Carbon-bonded, oxygen-bearing extractants; (ii) phosphorous-bonded, oxygen-bearing extractants; and (iii) high molecular weight aliphatic imines. The first two categories are based on the solvation of the acid by the donor bonds resulting in weak and nonspecific interactions between the acid and the solvent. Thus, they are not recommended for lactic acid extraction. In the third category, a specific reaction of proton transfer between the lactic acid and the imine occurs. This allows a further ion-pair interaction between the acid and the imine, bringing the lactic acid to the organic phase.
In extractive fermentation, these systems have a main drawback because of their toxic effect on organisms. Therefore, immobilization of the amine based compounds in solid sorbents has been used in lactic acid production. The low capacity of the resins, typically between 0.1-0.2 g lactic acid/g resin, and the fact that they are solid and must be continuously added to the fermentor as lactic acid is being synthesized, makes these processes difficult to operate.
Polyethyleneimine (PEI), has been used as a phase-forming polymer in ATPS. Ethylene oxide-propylene oxide random copolymers (EOPO) are phase-forming polymers when mixed with starch and are easily recyclable by temperature-induced phase separation and, thus, of great technical interest. As a drawback, both cells and carboxylic acids tend to partition to the starch rich phase of EOPO-Dextran (DEX) ATPS making the carboxylic acid recovery process inefficient.
Accordingly, there exists a need in the art for improved methods for recovering carboxylic acids, such as lactic acid, from solutions, particularly fermentation mixtures containing the acids. In addition, there exists a need in the art for reagents for facilitating the removal of carboxylic acids from such solutions.